This invention relates to photoresists and, more particularly, to photosensitive compositions, as well as other compositions useful in conjunction with photosensitive compositions, which provide flexible photoresist coatings for use in processes employing photoresists, e.g. the production of printing plates, metal plating, etching of metals or ceramics, as in the production of circuit boards or other electrical components, and other standard uses of photoresists.
Photoresists are generally photosensitive or lightsensitive materials which, when exposed to light of the proper wave length, are chemically changed in their solubility or removability characteristics in certain solvents or developers. To develop a photoresist generally means to remove, by whatever means, those areas of the photoresist which become more easily removable during the selective exposure through the mask. A common and convenient means of development is through solvents which dissolve the more easily removed portion of the selectively exposed photoresist layer, but leave the remaining portions substantially intact. A material is "developable" as used in this application, if it is removable by any of the standard methods of development.
Two types of photoresists are available, negativeacting and positive-acting resists. Prior to exposure, negative-acting resists are soluble in developer, but with light exposure they undergo chemical change and become insoluble in such developers. After selective exposure through a film pattern, the unexposed photoresist is selectively dissolved, softened, or washed away, leaving the desired resist pattern on a substrate. The remaining resist pattern is used to protect the substrate, e.g. in etching processes, where surface etching of the substrate is desired only in the areas which underlie the unexposed resist.
Positive-acting resists work in the opposite manner, with light exposure making the resists soluble in the developer. In either case, the resist pattern that remains after development (and post baking in some cases) is insoluble and chemically resistent to the cleaning, plating, etching or other solutions used in the process of treating the substrate.
Processes involving the utilization of photoresist coatings, such as in the formation of printed circuit boards, photolithographs, name plates and the like, are well known in the art. These materials are particularly useful in the formation of printed circuit boards, where a typical operation might be the coating of a metal clad plastic base sheet with a photoresist composition in a thin film, which is thereafter exposed through either a positive or negative of the desired image. If the photoresist used is a positive-acting material, the lightexposed areas of the resist layer are rendered soluble by the exposure, and are removed by washing with a developer to leave the underlying metal layer exposed. The area surrounding the exposed metal, which were not reached by light in the selective exposure, remain and protect the portions of the metal layer which they overlie. An etchant to which the resist is impervious may then be used to etch away the exposed metal areas, or selective plating or other processes may be accomplished, and there remains a layer of resist in the desired image pattern. Later on in the manufacturing process, the remaining resist pattern may or may not be removed, as desired.
Previous resist compositions, particularly the positive-acting diazo-type resists, have been difficult to work with in order to obtain a quality product. Such diazo materials, which are normally incorporated in an alkali soluble resin such as phenol or cresol-formaldehyde novolak resins, are disclosed, for example, in U.S. Pat. No. 3,046,118, incorporated herein by reference, as well as in other patents, including U.S. Pat. Nos. 3,102,809; 3,106,465; 3,130,047; 3,130,048; 3,148,983, 3,061,430; 3,184,310; 3,188,210; 3,201,239; and 3,288,608. Such materials are typically highly brittle, and while in the liquid state have poor rheology or flow characteristics, do not wet the surface well, and exhibit poor drying characteristics. The poor rheology or inability to flow properly leads to "pinhole" defects, which are simply small points on the surface of the substrate which the photoresist has failed to completely flow over and wet. The brittleness leads to difficulties since whenever there is any undercutting, i.e. removal of the part of the resist layer close to the substrate by the etchant or other material being used on the substrate, or by the developer for the developable resist material, there remains a thin, brittle, unsupported layer of the resist, which may break off when the substrate is exposed to washing or rinsing operations. As a result, significant portions of the substrate surface become unprotected, and future operations such as etching, metal deposition, etc., take place on unwanted areas of the substrate.
Photoresist materials have previously been applied in a number of ways, either as a liquid or as a dry film. Liquids are typically applied by using rollers, wicks or squeegees, by dipping, by spraying or wiping, etc., the liquid photoresist material across the surface of the substrate to form a layer of liquid resist on the base material, followed by solidification of the resist.
Particular problems arise in connection with the photoresist production of printed circuit boards, because of the necessity of providing through-holes in the substrate, in order to obtain conductivity between the top of the substrate and the other side of the substrate. These through-holes are simply holes through the substrate and any metal cladding thereon, which are typically catalyzed and plated with electroless plating solution in order to form an electrical connection between the surfaces of the substrate.
When liquid materials are used in connection with the preparation of printed circuit boards having through-holes, the resist in the liquid form is frequently forced into the through-holes, where it (a) may not become sufficiently exposed to the activating radiation to become soluble, or (b) may be protected from the developer whereby it is not dissolved or removed in a reasonable time from the board. Should either of these occur, the presence of a residuum or resist in the through-holes prevents proper copper plating therein, and often results in defective printed circuits.
The difficulties of liquid resists in plugging through-holes gave impetus to the development of dry film methods of application of photoresist materials. See, e.g., U.S. Pat. Nos. 3,526,504 and 3,629,036, both of which are incorporated herein by reference. Dry film photoresists are made by forming a coating of photosensitive resist material on a base sheet, e.g. plastic such as polyethylene, polypropylene or polyesters, e.g. duPont Mylar, and the other side of the photosensitive resist material is usually also covered with a protective sheet, particularly if the photosensitive material is adversely affected by air, as are most negative-acting materials. Dry film resists may be supplied as sheets or rolls, and are typically applied to the substrate by removing the protective or backing layer, and adhering by heat and pressure the dry film layer of photoresist to the substrate.
Another significant development along this line is disclosed in copending U.S. patent application Ser. No. 655,082 filed Mar. 8, 1976, now abandoned, incorporated herein by reference, which discloses the use of a thin layer of photosensitive resist, preferably in the form of a dry film, overlaying a layer of nonphotosensitive, but developable resist. For example, such a dry film might comprise an upper layer of photoresist containing a diazo activator and an alkali soluble novolak resin, over a layer of non-photosensitive developable material, e.g. simply novolak resin without activator.
The development of dry film applied photoresists did much to eradicate the plugging of through-holes by the photoresist. However, many of the remaining problems were not solved. Since the dry film photoresist must be handled, even rolled, prior to application to the substrate, brittleness is even more of a problem, particularly with the diazo-type positive-acting photoresist mentioned above. Bending of sheets of very brittle materials causes cracking and flaking of the photoresist, which can result in inadequate coverage on the substrate. Also, pinholes and other defects caused by the photosensitive resist's inability to properly flow and coat the base sheet from which it is transferred to the substrate, can also transfer to the substrate, i.e., pinholes resulting when the liquid photoresist is formed into a layer on the plastic base sheet, can be transferred to the substrate when the photoresist is adhered to that substrate.
Attempts have been made to improve the characteristics of the photoresist materials by inclusion of film-forming materials to form thick coatings, e.g. in U.S. Pat. No. 3,402,044, incorporated herein by reference, and more recently by the incorporation of polyvinyl ethers into the photoresist material, U.S. Pat. No. 3,634,082, incorporated herein by reference. However, while some improvements were obtained, many of the previous problems remained unsolved.
It is accordingly an object of the present invention to provide a photoresist material which does not suffer from the characteristics of brittleness and poor rheology suffered by previous photoresist materials. It is a further object of the present invention to provide a photoresist material which is highly flexible and can be readily bent or worked with without cracking, flaking or peeling. It is a further object to provide flexible photoresist materials with improved flow characteristics. It is a further object to provide an additive for diazo-type photoresist compositions, which improves flexibility and flow properties, and decreases point defects and other imperfections.